/* Copyright 2005-2006 Tim Fennell
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package jpa4azure.util;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.lang.annotation.Annotation;
import java.net.URL;
import java.net.URLDecoder;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Set;
import java.util.jar.JarEntry;
import java.util.jar.JarInputStream;

/**
 * <p>
 * ResolverUtil is used to locate classes that are available in the/a class path
 * and meet arbitrary conditions. The two most common conditions are that a
 * class implements/extends another class, or that is it annotated with a
 * specific annotation. However, through the use of the {@link Test} class it is
 * possible to search using arbitrary conditions.
 * </p>
 * 
 * <p>
 * A ClassLoader is used to locate all locations (directories and jar files) in
 * the class path that contain classes within certain packages, and then to load
 * those classes and check them. By default the ClassLoader returned by {@code
 * Thread.currentThread().getContextClassLoader()} is used, but this can be
 * overridden by calling {@link #setClassLoader(ClassLoader)} prior to invoking
 * any of the {@code find()} methods.
 * </p>
 * 
 * <p>
 * General searches are initiated by calling the
 * {@link #find(Test1.sourceforge.stripes.util.ResolverUtil.Test, String)} ()}
 * method and supplying a package name and a Test instance. This will cause the
 * named package <b>and all sub-packages</b> to be scanned for classes that meet
 * the test. There are also utility methods for the common use cases of scanning
 * multiple packages for extensions of particular classes, or classes annotated
 * with a specific annotation.
 * </p>
 * 
 * <p>
 * The standard usage pattern for the ResolverUtil class is as follows:
 * </p>
 * 
 *<pre>
 * esolverUtil&lt;ActionBean&gt; resolver = new ResolverUtil&lt;ActionBean&gt;();
 * esolver.findImplementation(ActionBean.class, pkg1, pkg2);
 * esolver.find(new CustomTest(), pkg1);
 * esolver.find(new CustomTest(), pkg2);
 * ollection&lt;ActionBean&gt; beans = resolver.getClasses();
 *</pre>
 * 
 * @author Tim Fennell
 */
public class ResolverUtil<T> {
	/** An instance of Log to use for logging in this class. */

	/**
	 * A simple interface that specifies how to test classes to determine if
	 * they are to be included in the results produced by the ResolverUtil.
	 */
	public static interface Test {
		/**
		 * Will be called repeatedly with candidate classes. Must return True if
		 * a class is to be included in the results, false otherwise.
		 */
		boolean matches(Class<?> type);
	}

	/**
	 * A Test that checks to see if each class is assignable to the provided
	 * class. Note that this test will match the parent type itself if it is
	 * presented for matching.
	 */
	public static class IsA implements Test {
		private Class<?> parent;

		/**
		 * Constructs an IsA test using the supplied Class as the parent
		 * class/interface.
		 */
		public IsA(Class<?> parentType) {
			this.parent = parentType;
		}

		/**
		 * Returns true if type is assignable to the parent type supplied in the
		 * constructor.
		 */
		@SuppressWarnings("unchecked")
		public boolean matches(Class type) {
			return type != null && parent.isAssignableFrom(type);
		}

		@Override
		public String toString() {
			return "is assignable to " + parent.getSimpleName();
		}
	}

	/**
	 * A Test that checks to see if each class is annotated with a specific
	 * annotation. If it is, then the test returns true, otherwise false.
	 */
	public static class AnnotatedWith implements Test {
		private Class<? extends Annotation> annotation;

		/** Constructs an AnnotatedWith test for the specified annotation type. */
		public AnnotatedWith(Class<? extends Annotation> annotation) {
			this.annotation = annotation;
		}

		/**
		 * Returns true if the type is annotated with the class provided to the
		 * constructor.
		 */
		@SuppressWarnings("unchecked")
		public boolean matches(Class type) {
			return type != null && type.isAnnotationPresent(annotation);
		}

		@Override
		public String toString() {
			return "annotated with @" + annotation.getSimpleName();
		}
	}

	/** The set of matches being accumulated. */
	private Set<Class<? extends T>> matches = new HashSet<Class<? extends T>>();

	/**
	 * The ClassLoader to use when looking for classes. If null then the
	 * ClassLoader returned by Thread.currentThread().getContextClassLoader()
	 * will be used.
	 */
	private ClassLoader classloader;

	/**
	 * Provides access to the classes discovered so far. If no calls have been
	 * made to any of the {@code find()} methods, this set will be empty.
	 * 
	 * @return the set of classes that have been discovered.
	 */
	public Set<Class<? extends T>> getClasses() {
		return matches;
	}

	/**
	 * Returns the classloader that will be used for scanning for classes. If no
	 * explicit ClassLoader has been set by the calling, the context class
	 * loader will be used.
	 * 
	 * @return the ClassLoader that will be used to scan for classes
	 */
	public ClassLoader getClassLoader() {
		return classloader == null ? Thread.currentThread()
				.getContextClassLoader() : classloader;
	}

	/**
	 * Sets an explicit ClassLoader that should be used when scanning for
	 * classes. If none is set then the context classloader will be used.
	 * 
	 * @param classloader
	 *            a ClassLoader to use when scanning for classes
	 */
	public void setClassLoader(ClassLoader classloader) {
		this.classloader = classloader;
	}

	/**
	 * Attempts to discover classes that are assignable to the type provided. In
	 * the case that an interface is provided this method will collect
	 * implementations. In the case of a non-interface class, subclasses will be
	 * collected. Accumulated classes can be accessed by calling
	 * {@link #getClasses()}.
	 * 
	 * @param parent
	 *            the class of interface to find subclasses or implementations
	 *            of
	 * @param packageNames
	 *            one or more package names to scan (including subpackages) for
	 *            classes
	 */
	public ResolverUtil<T> findImplementations(Class<?> parent,
			String... packageNames) {
		if (packageNames == null)
			return this;

		Test test = new IsA(parent);
		for (String pkg : packageNames) {
			find(test, pkg);
		}

		return this;
	}

	/**
	 * Attempts to discover classes that are annotated with the annotation.
	 * Accumulated classes can be accessed by calling {@link #getClasses()}.
	 * 
	 * @param annotation
	 *            the annotation that should be present on matching classes
	 * @param packageNames
	 *            one or more package names to scan (including subpackages) for
	 *            classes
	 */
	public ResolverUtil<T> findAnnotated(
			Class<? extends Annotation> annotation, String... packageNames) {
		if (packageNames == null)
			return this;

		Test test = new AnnotatedWith(annotation);
		for (String pkg : packageNames) {
			find(test, pkg);
		}

		return this;
	}

	/**
	 * Scans for classes starting at the package provided and descending into
	 * subpackages. Each class is offered up to the Test as it is discovered,
	 * and if the Test returns true the class is retained. Accumulated classes
	 * can be fetched by calling {@link #getClasses()}.
	 * 
	 * @param test
	 *            an instance of {@link Test} that will be used to filter
	 *            classes
	 * @param packageName
	 *            the name of the package from which to start scanning for
	 *            classes, e.g. {@code net.sourceforge.stripes}
	 */
	public ResolverUtil<T> find(Test test, String packageName) {
		packageName = packageName.replace('.', '/');
		ClassLoader loader = getClassLoader();
		Enumeration<URL> urls;

		try {
			urls = loader.getResources(packageName);
		} catch (IOException ioe) {
			return this;
		}

		while (urls.hasMoreElements()) {
			String urlPath = urls.nextElement().getFile();
			try {
				urlPath = URLDecoder.decode(urlPath, "UTF-8");
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
			}

			// If it's a file in a directory, trim the stupid file: spec
			if (urlPath.startsWith("file:")) {
				urlPath = urlPath.substring(5);
			}

			// Else it's in a JAR, grab the path to the jar
			if (urlPath.indexOf('!') > 0) {
				urlPath = urlPath.substring(0, urlPath.indexOf('!'));
			}

			File file = new File(urlPath);
			if (file.isDirectory()) {
				loadImplementationsInDirectory(test, packageName, file);
			} else {
				loadImplementationsInJar(test, packageName, file);
			}
		}

		return this;
	}

	/**
	 * Finds matches in a physical directory on a filesystem. Examines all files
	 * within a directory - if the File object is not a directory, and ends with
	 * <i>.class</i> the file is loaded and tested to see if it is acceptable
	 * according to the Test. Operates recursively to find classes within a
	 * folder structure matching the package structure.
	 * 
	 * @param test
	 *            a Test used to filter the classes that are discovered
	 * @param parent
	 *            the package name up to this directory in the package
	 *            hierarchy. E.g. if /classes is in the classpath and we wish to
	 *            examine files in /classes/org/apache then the values of
	 *            <i>parent</i> would be <i>org/apache</i>
	 * @param location
	 *            a File object representing a directory
	 */
	private void loadImplementationsInDirectory(Test test, String parent,
			File location) {
		File[] files = location.listFiles();
		StringBuilder builder = null;

		// File.listFiles() can return null when an IO error occurs!
		if (files == null) {
			return;
		}

		for (File file : files) {
			builder = new StringBuilder(100);
			builder.append(parent).append("/").append(file.getName());
			String packageOrClass = (parent == null ? file.getName() : builder
					.toString());

			if (file.isDirectory()) {
				loadImplementationsInDirectory(test, packageOrClass, file);
			} else if (file.getName().endsWith(".class")) {
				addIfMatching(test, packageOrClass);
			}
		}
	}

	/**
	 * Finds matching classes within a jar files that contains a folder
	 * structure matching the package structure. If the File is not a JarFile or
	 * does not exist a warning will be logged, but no error will be raised.
	 * 
	 * @param test
	 *            a Test used to filter the classes that are discovered
	 * @param parent
	 *            the parent package under which classes must be in order to be
	 *            considered
	 * @param jarfile
	 *            the jar file to be examined for classes
	 */
	private void loadImplementationsInJar(Test test, String parent, File jarfile) {

		try {
			JarEntry entry;
			JarInputStream jarStream = new JarInputStream(new FileInputStream(
					jarfile));

			while ((entry = jarStream.getNextJarEntry()) != null) {
				String name = entry.getName();
				if (!entry.isDirectory() && name.startsWith(parent)
						&& name.endsWith(".class")) {
					addIfMatching(test, name);
				}
			}
		} catch (IOException ioe) {
			ioe.printStackTrace();
		}
	}

	/**
	 * Add the class designated by the fully qualified class name provided to
	 * the set of resolved classes if and only if it is approved by the Test
	 * supplied.
	 * 
	 * @param test
	 *            the test used to determine if the class matches
	 * @param fqn
	 *            the fully qualified name of a class
	 */
	@SuppressWarnings("unchecked")
	protected void addIfMatching(Test test, String fqn) {
		try {
			String externalName = fqn.substring(0, fqn.indexOf('.')).replace(
					'/', '.');
			ClassLoader loader = getClassLoader();

			Class type = loader.loadClass(externalName);
			if (test.matches(type)) {
				matches.add((Class<T>) type);
			}
		} catch (Throwable t) {
			t.printStackTrace();
		}
	}
}
